角斗士？

Gladiator?

Capítulo 1 ¿Gladiador?

Li Yue traveled to the devil's castle, and he was not a cool vampire or a mysterious magician, but a bald brain spirit?
  Fortunately, the brain spirit has the ability to perceive thoughts, move objects through the air, and shock souls... at least it can be considered a magical creature.
  Li Yue, who has unlocked the "Demon's Encyclopedia", will start as a brain spirit, collect strange monsters, transform the inner demons, summon monsters from other worlds... wander in chaos and darkness, and climb to the top of the Demon King's City step by step.